The present invention relates to a new and improved method and apparatus for manufacturing thermoelectric devices of the type utilized in the generation of electricity from heat.
It has been recognized that the world supply of fossil fuels for the production of energy is being exhausted at ever increasing rates. This realization has resulted in an energy crisis which impacts not only the world's economy, but threatens the peace and stability of the world. The solution to the energy crisis lies in the development of new fuels and more efficient techniques to utilize them. To that end, the present invention deals with energy conservation, power generation, pollution, and the generation of new business opportunities by the development of new methods and apparatuses for commercially producing thermoelectric devices which provide more electricity.
An important part of the solution with respect to the development of permanent, economical energy conversion lies in the field of thermoelectrics wherein electrical power is generated from heat. It has been estimated that more than two-thirds of all our energy, for example, from automobile exhausts or power plants, is wasted and given off to the environment. Up until now, there has been no serious climatic effect from this thermal pollution. However, it has been predicted that as the world's energy consumption increases, the effects of thermal pollution will ultimately lead to a partial melting of the polar ice caps with an attendant increase in sea level.
The efficiency of a thermoelectric device can be expressed in terms of a figure of merit (Z) for the material forming the device, wherein Z is defined as: EQU Z=S.sup.2 .sigma./K
Where:
Z is expressed in units.times.10.sup.3 /.degree.C. PA1 S is the Seebeck coefficient in .mu.V/.degree.C. PA1 K is the thermal conductivity in mW/cm-.degree.C. PA1 .sigma. is the electrical conductivity in (.OMEGA.-cm).sup.-1
From the above, one can see that in order for a material to be suitable for thermoelectric power conversion, it must have a large value for the thermoelectric power Seebeck coefficient (S), a high electrical conductivity (.sigma.), and a low thermal conductivity (K). Further, there are two components to the thermal conductivity (K):K.sub.1, the lattice component; and K.sub.e, the electrical component. In non-metals, K.sub.1 dominates and it is this component which mainly determines the value of K.
Stated in another way, in order for a material to be efficient for thermoelectric power conversion, it is important to allow carriers to diffuse easily from the hot junction to the cold junction while maintaining the temperature gradient. Hence, high electrical conductivity is required along with low thermal conductivity.
Thermoelectric power conversion has not found wide usage in the past. The major reason for this is that prior art thermoelectric materials which are at all suitable for commercial applications have been crystalline in structure. Crystalline solids cannot attain large values of electrical conductivity while maintaining low thermal conductivity. Most importantly, because of crystalline symmetry, thermal conductivity cannot be controlled by modification.
In the case of the conventional polycrystalline approach, the problems of single crystalline materials still dominate. However, new problems are also encountered by virtue of the polycrystalline grain boundaries which cause these materials to have relatively low electrical conductivities. In addition, the fabrication of these materials is also difficult to control as a result of their more complex crystalline structure. The chemical modification or doping of these materials, because of the above problems are especially difficult.
Among the best known currently existing polycrystalline thermoelectric materials are (Bi,Sb).sub.2 Te.sub.3, PbTe, and Si-Ge. The (Bi,Sb).sub.2 Te.sub.3 materials are best suited for applications in the -10.degree. C.+150.degree. C. range with its best Z appearing at around 30.degree. C. (Bi,Sb).sub.2 Te.sub.3 represents a continuous solid solution system in which the relative amount of Bi and Sb are from 0 to 100%. The Si-Ge material is best suited for high temperature applications in the 600.degree. C. to 1000.degree. C. range with a satisfactory Z appearing at above 700.degree. C. The PbTe polycrystalline material exhibits its best figure of merit in the 300.degree. C. to 500.degree. C. range. None of these materials is well suited for applications in the 100.degree. C. to 300.degree. C. range. This is indeed unfortunate, because it is in this temperature range where a wide variety of waste heat applications are found. Among such applications are geothermal waste heat and waste heat from internal combustion engines in, for example, trucks, buses, and automobiles. Applications of this kind are important because the heat is truly waste heat. Heat in the higher temperature ranges must be intentionally generated with other fuels and therefore is not truly waste heat.
New and improved thermoelectric alloy materials have been discovered for use in the aforesaid temperature ranges. These materials are disclosed and claimed in copending U.S. application Ser. No. 341,864, filed Jan. 22, 1982 in the names of Tumkur S. Jayadev and On Van Nguyen for NEW MULTIPHASE THERMOELECTRIC ALLOYS AND METHOD OF MAKING SAME, now abandoned in favor of continuation-in-part application Ser. No. 412,306, filed Jan. 17, 1983 and allowed Sept. 26, 1983, which application is incorporated herein by reference.
The thermoelectric materials there disclosed can be utilized in the thermoelectric devices manufactured by the method and apparatus herein. These materials are not single phase crystalline materials, but instead, are disordered materials. Further, these materials are multiphase materials having both amorphous and multiple crystalline phases. Materials of this type are good thermal insulators. They include grain boundaries of various transitional phases varying in composition from the composition of matrix crystallites to the composition of the various phases in the grain boundary regions. The grain boundaries are highly disordered with the transitional phases including phases of high thermal resistivity to provide high resistance to thermal conduction. Contrary to conventional materials, the material is designed such that the grain boundaries define regions including conductive phases therein providing numerous electrical conduction paths through the bulk material for increasing electrical conductivity without substantially effecting the thermal conductivity. In essence, these materials have all of the advantages of polycrystalline materials in desirably low thermal conductivities and crystalline bulk Seebeck properties. However, unlike the conventional polycrystalline materials, these disordered multiphase materials also have desirably high electrical conductivities. Hence, as disclosed in the aforesaid referenced application, the S.sup.2 .sigma. product for the figure of merit of these materials can be independently maximized with desirably low thermal conductivities for thermoelectric power generation.
Amorphous materials, representing the highest degree of disorder, have been made for thermoelectric applications. The materials and methods for making the same are fully disclosed and claimed, for example, in U.S. Pat. Nos. 4,177,473, 4,177,474, and 4,178,415 which issued in the name of Stanford R. Ovshinsky. The materials disclosed in these patents are formed in a solid amorphous host matrix having structural configurations which have local rather than long-range order and electronic configurations which have an energy gap and an electrical activation energy. Added to the amorphous host matrix is a modifier material having orbitals which interact with the amorphous host matrix as well as themselves to form electronic states in the energy gap. This interaction substantially modifies the electronic configurations of the amorphous host matrix to substantially reduce the activation energy and hence, increase substantially the electrical conductivity of the material. The resulting electrical conductivity can be controlled by the amount of modifier material added to the host matrix. The amorphous host matrix is normally of intrinsic-like conduction and the modified material changes the same to extrinsic-like conduction.
As also disclosed therein, the amorphous host matrix can have lone-pairs having orbitals wherein the orbitals of the modifier material interact therewith to form the new electronic states in the energy gap. In another form, the host matrix can have primarily tetrahedral bonding wherein the modifier material is added primarily in a non-substitutional manner with its orbitals interacting with the host matrix. Both d and f band materials as well as boron and carbon, which add multiorbital possibilities can be used as modifiers to form the new electronic states in the energy gap.
As a result of the foregoing, these amorphous thermoelectric materials have substantially increased electrical conductivity. However, because they remain amorphous after modification, they retain their low thermal conductivities making them well suited for thermoelectric applications, especially in high temperature ranges above 400.degree. C.
These materials are modified on an atomic or microscopic level with the atomic configurations thereof substantially changed to provide the heretofore mentioned independently increased electrical conductivities. In contrast, the materials disclosed in the aforesaid referenced application are not atomically modified. Rather, they are fabricated in a manner which introduces disorder into the material on a macroscopic level. This disorder allows various phases including conductive phases to be introduced into the material much in the same manner as modification atomically in pure amorphous phase materials to provide controlled high electrical conductivity while the disorder in the other phases provides low thermal conductivity. These materials therefore are intermediate in terms of their thermal conductivity between amorphous and regular polycrystalline materials.
A thermoelectric device generates electricity by the establishment of a temperature differential across the materials contained therein. The thermoelectric devices generally include elements of both p-type and n-type material. In the p-type material the temperature differential drives positively charged carriers from the hot side to the cold side of the elements, while in the n-type material the temperature differential drives negatively charged carriers from the hot side to the cold side of the elements.
Thermoelectric power conversion has not found wide usage in the past not only because of material limitations but also because of device limitations. One serious limitation exhibited by such devices results from the use of relatively thick ceramic substrates. The substrates, because of their coefficient of thermal expansion, bow or warp during manufacture and subsequent use. This results in heat transfer losses between the devices and their associated heat exchanger because the warping precludes broad surface contact therebetween. Also the substrates add significant thermal resistance in series with the thermoelectric elements of the devices to further increase heat transfer losses.
A new and improved thermoelectric device has been discovered overcoming the above-stated device limitations. This device and a method of making same is disclosed and claimed in co-pending U.S. application Ser. No. 372,688, filed Apr. 28, 1982 in the name of Der-Jeou Chou for IMPROVED THERMOELECTRIC DEVICE AND METHOD OF MAKING SAME, allowed Mar. 7, 1984, which application is incorporated herein by reference.
As disclosed therein, bowing or warping of the device is prevented during manufacture and use thereof. This device configuration not only eliminates the need for the ceramic substrates but also minimizes the effects of thermal expansion of the other device components by both evenly distributing and absorbing the expansion throughout the device that does occur. Thus, broad surface contact between the device and a heat exchanger can be readily maintained when utilized in a thermoelectric system. Because the device there disclosed eliminates the ceramic substrates, the thermal resistance in series with the device elements is also reduced to allow a greater temperature differential to exist across the thermoelectric elements for increasing the obtainable electric power output for a given total temperature differential.
The cost effectiveness of thermoelectric power conversion and its acceptance in the market place is further influenced, not only by efficient thermoelectric materials and devices as discussed above, but also by the ability to produce the devices on a commercial cost effective scale. The present invention provides such a method and apparatus for mass producing thermoelectric devices on a cost effective, commercial scale.